Photo-induced formation of dissolved gaseous mercury in coastal and offshore seawater of the Mediterranean basin

Superoxide-Mediated Extracellular Mercury Reduction by Aerobic Marine Bacterium Alteromonas sp. KD01

Microbial reduction plays a crucial role in Hg redox and the global cycle. Although intracellular Hg(II) reduction mediated by MerA protein is well documented, it is still unclear whether or how bacteria reduce Hg(II) extracellularly without its internalization. Herein, for the first time, we discovered the extracellular reduction of Hg(II) by a widely distributed aerobic marine bacterium Alteromonas sp. KD01 through a superoxide-dependent mechanism. The generation of superoxide by Alteromonas sp. KD01 was determined using 3-bis(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide and methyl cypridina luciferin analogue as probes via UV-vis and chemiluminescence detection, respectively. The results demonstrated that Hg(II) reduction was inhibited by superoxide scavengers (superoxide dismutase (SOD) and Cu(NO3)2) or inhibitors of reduced nicotinamide adenine dinucleotide (NADH) oxidoreductases. In contrast, the addition of NADH significantly improved superoxide generation and, in turn, Hg(II) reduction. Direct evidence of superoxide-mediated Hg(II) reduction was provided by the addition of superoxide using KO2 in deionized water and seawater. Moreover, we observed that even superoxide at an environmental concentration of 9.6 ± 0.5 nM from Alteromonas sp. KD01 (5.4 × 106 cells mL-1) was capable of significantly reducing Hg(II). Our findings provide a greater understanding of Hg(II) reduction by superoxide from heterotrophic bacteria and eukaryotic phytoplankton in diverse aerobic environments, including surface water, sediment, and soil.

Mercury (Hg) in the "Skin" of the Ocean: Dissolved Gaseous Hg, Total Hg, and Hg Redox Chemistry in Sea Surface Microlayer and Implication for Air-Sea Hg Exchange

The sea surface microlayer (SML) is the uppermost ∼1000 μm of the surface of the ocean. With distinct physicochemical properties and position relative to the adjacent subsurface waters (SSWs), the ubiquitous distribution and high dynamics of the SML greatly regulate the global air-sea gas exchange and biogeochemistry. Mercury (Hg) redox chemistry in surface seawaters and air-sea exchange of gaseous Hg (mainly Hg(0)) fundamentally control the global oceanic Hg cycle. However, the occurrence and transformation of Hg in the SML have been poorly quantified. Here we optimize the traditional SML sampling system to make it more suitable for dissolved gaseous Hg (DGM, mainly Hg(0)) sampling. We then assess the temporal and spatial variability of DGM, total Hg, dissolved organic carbon (DOC), and Hg redox chemistry in the SML and SSWs of diverse marine environments. Our data suggest a general DGM, total Hg, and DOC enrichment in the SML relative to the SSWs but with complex variability in time and space. The incubation experiments further reveal the complex characteristics of Hg redox chemistry between the SML and SSWs. We discuss important implications of the SML Hg cycle on air-sea Hg exchange and suggest wider investigations of the SML Hg cycle in the global hydrosphere.

Dissolved gaseous mercury production and sea-air gaseous exchange in impacted coastal environments of the northern Adriatic Sea

The northern Adriatic Sea is well known for mercury (Hg) contamination mainly due to historical Hg mining which took place in Idrija (Slovenia). The formation of dissolved gaseous mercury (DGM) and its subsequent volatilisation can reduce the amount of Hg available in the water column. In this work, the diurnal patterns of both DGM production and gaseous Hg fluxes at the water-air interface were seasonally evaluated in two selected environments within this area, a highly Hg-impacted, confined fish farm (VN: Val Noghera, Italy) and an open coastal zone less impacted by Hg inputs (PR: Bay of Piran, Slovenia). A floating flux chamber coupled with real-time Hg⁰ analyser was used for flux estimation in parallel with DGM concentrations determination through in-field incubations. Substantial DGM production was observed at VN (range = 126.0-711.3 pg L^-1) driven by both strong photoreduction and possibly dark biotic reduction, resulting in higher values in spring and summer and comparable concentrations throughout both day and night. Significantly lower DGM was observed at PR (range = 21.8-183.4 pg L^-1). Surprisingly, comparable Hg⁰ fluxes were found at the two sites (range VN = 7.43-41.17 ng m^-2 h^-1, PR = 0-81.49 ng m^-2 h^-1), likely due to enhanced gaseous exchanges at PR thanks to high water turbulence and to the strong limitation of evasion at VN by water stagnation and expected high DGM oxidation in saltwater. Slight differences between the temporal variation of DGM and fluxes indicate that Hg evasion is more controlled by factors such as water temperature and mixing conditions than DGM concentrations alone. The relative low Hg losses through volatilisation at VN (2.4-4.6% of total Hg) further confirm that static conditions in saltwater environments negatively affect the ability of this process in reducing the amount of Hg retained in the water column, therefore potentially leading to a greater availability for methylation and trophic transfer.

Photochemical behaviors of mercury (Hg) species in aquatic systems: A systematic review on reaction process, mechanism, and influencing factor

The fate and transport of Hg species in natural aquatic environment are strongly affected by photochemical transformation of Hg⁰, Hg²⁺, and MeHg. Migration of Hg is determined by its complexation with organic and inorganic ligands that are widely present in the water. The presence of dissolved organic matter (DOM) is closely related to photochemical reactions of Hg. DOM can strongly bind to mercury (e.g., Hg²⁺ and MeHg), thus affecting its speciation, mobility and toxicity, eventually dominating its bioavailability. This review summarizes extensive studies on photochemical behaviors of Hg including: (1) photo-oxidation; (2) photo-reduction; (3) photochemical methylation; and (4) MeHg photo-degradation. Photo-oxidation of Hg⁰ is mostly caused by oxidative free radicals (e.g., •OH, CO₃^•-, O₃, and ¹O₂), while photo-reduction of Hg²⁺ is more complicated and it involves two pathways: (1) primary processes (direct photolysis of Hg²⁺ or ligand-metal charge transfer of Hg²⁺-DOM complex); and (2) secondary processes (reduction of Hg²⁺-DOM complex induced by free radicals derived from DOM photolysis). Photochemical methylation of inorganic Hg occurs as follows: (1) Hg²⁺ complexes with methyl donors (e.g., acetic acid, tert-butyl, alcohols, etc.) to form intermediates, followed by (2) an intramolecular methyl transfer. MeHg photo-degradation is the leading pathway for MeHg demethylation and it primarily proceeds via four different pathways. The information on DOM was also mentioned, but DOM is not the only factor that affects the photochemical behaviors of Hg. Other influencing factors such as: (1) pH value; (2) dissolved oxygen; (3) cations (Fe³⁺, K⁺) and anions (NO₃^-, HCO₃^-, CO₃^2-, Cl^-); and (4) suspended substance cannot be ignored.

Characteristics of mercury speciation in seawater and emission flux of gaseous mercury in the Bohai Sea and Yellow Sea

Four cruises were performed in the Bohai Sea (BS) and Yellow Sea (YS) to ascertain the levels and distributions of gaseous elemental mercury (GEM), dissolved gaseous mercury (DGM), methylmercury (MeHg), and total mercury (THg) during 2012 and 2014. Their concentrations and Hg⁰ flux exhibited clear spatial-temporal distributions. The GEM level over the BS in spring (2.71 ± 0.49 ng m^-3) was significantly higher than that in fall (1.98 ± 0.91 ng m^-3). Air masses with elevated GEM mainly originated from northern China. During the two cruises in 2012 over the BS, the mean DGM concentration in spring (35.7 ± 4.6 pg l^-1) was comparable to that in fall (32.4 ± 4.6 pg l^-1). During the spring cruise of 2014, the mean DGM concentration in the BS (52.8 ± 12.5 pg l^-1) was comparable to that in the YS (52.4 ± 14.1 pg l^-1), while during the fall cruise of 2014, it was significantly lower in the BS (26.7 ± 14.4 pg l^-1) than in the YS (57.2 ± 17.9 pg l^-1). DGM represents a small portion of unfiltered THg in the BS (3.95%) and YS (5.12%). The MeHg and MeHg% values were higher in nearshore areas than in open sea, indicating higher productivity in coastal regions. The Hg⁰ flux in the YS (4.56 ng m^-2 h^-1) was about twice that in the BS. The annual emission Hg⁰ fluxes from the BS and YS were 2.71 and 23.68 tons yr^-1, respectively.

Diurnal fluxes of gaseous elemental mercury from the water-air interface in coastal environments of the northern Adriatic Sea

A crucial step towards developing a more complete understanding of mercury (Hg) biogeochemical cycling in coastal environments is the measurement of the fluxes of gaseous elemental mercury (GEM), at the water-air interface (W-A interface). A floating flux chamber coupled with a real-time atomic adsorption spectrometer (Lumex-RA 915M) was applied to measure GEM concentrations, and to estimate the diurnal evasion flux at the W-A interface during three seasonal campaigns at four selected sites: two in a lagoon-based fish farm, one in an open lagoon environment highly impacted by long-term activities from the Idrija mercury mine (Slovenia), and an uncontaminated area of the Gulf of Trieste, the Bay of Piran (Slovenia). In this study, the regional background concentration measured at the uncontaminated site of atmospheric GEM (from 1.60 ± 0.95 to 2.87 ± 1.52 ng m^-3) was determined. GEM fluxes at the W-A interface were found to be significantly higher during the summer (from 51.2 ± 8.8 ng m^-2 h^-1 to 79.9 ± 11.4 ng m^-2 h^-1) and correlated to incident solar radiation and water temperature. This finding confirms the importance of these two parameters in the photoreduction and biotic reduction of Hg²⁺ to dissolved gaseous mercury (DGM), which is volatile and easily released to the atmosphere in the form of GEM. These new insights will be of help for future estimates of Hg mass balance in one of the most contaminated areas in the Adriatic Sea.

An insight into mercury reduction process by humic substances in aqueous medium under dark condition

Mercury (Hg) reduction by humic substances (HS) in the aquatic medium under the dark condition is a poorly understood but important process in Hg biogeochemical cycling. In this study, an effort was made to provide a better understanding of Hg(II) reduction by well-characterized humic substances under dark condition. Reduction of Hg(II) by dissolved HS in aquatic systems increases with increasing Hg loading. However, Hg(II) reduction gradually decreases with the increasing total S content and oxygen containing functional groups in the dissolved HS under dark condition. Increasing major cation concentration decreases the rate of Hg(II) reduction in aquatic systems. High concentration of Ca²⁺ ion slows down the intermolecular electron transfer from HS to Hg(II) and inhibits the formation of Hg⁰ in absence of light. This study indicates that complexation of Hg(II) and HS is essential for Hg reduction under dark condition.

Air-sea exchange of gaseous mercury in the tropical coast (Luhuitou fringing reef) of the South China Sea, the Hainan Island, China

The air-sea exchange of gaseous mercury (mainly Hg(0)) in the tropical ocean is an important part of the global Hg biogeochemical cycle, but the related investigations are limited. In this study, we simultaneously measured Hg(0) concentrations in surface waters and overlaying air in the tropical coast (Luhuitou fringing reef) of the South China Sea (SCS), Hainan Island, China, for 13 days on January-February 2015. The purpose of this study was to explore the temporal variation of Hg(0) concentrations in air and surface waters, estimate the air-sea Hg(0) flux, and reveal their influencing factors in the tropical coastal environment. The mean concentrations (±SD) of Hg(0) in air and total Hg (THg) in waters were 2.34 ± 0.26 ng m(-3) and 1.40 ± 0.48 ng L(-1), respectively. Both Hg(0) concentrations in waters (53.7 ± 18.8 pg L(-1)) and Hg(0)/THg ratios (3.8 %) in this study were significantly higher than those of the open water of the SCS in winter. Hg(0) in waters usually exhibited a clear diurnal variation with increased concentrations in daytime and decreased concentrations in nighttime, especially in cloudless days with low wind speed. Linear regression analysis suggested that Hg(0) concentrations in waters were positively and significantly correlated to the photosynthetically active radiation (PAR) (R (2) = 0.42, p < 0.001). Surface waters were always supersaturated with Hg(0) compared to air (the degree of saturation, 2.46 to 13.87), indicating that the surface water was one of the atmospheric Hg(0) sources. The air-sea Hg(0) fluxes were estimated to be 1.73 ± 1.25 ng m(-2) h(-1) with a large range between 0.01 and 6.06 ng m(-2) h(-1). The high variation of Hg(0) fluxes was mainly attributed to the greatly temporal variation of wind speed.

Air-sea exchange of gaseous mercury in the East China Sea

Two oceanographic cruises were carried out in the East China Sea (ECS) during the summer and fall of 2013. The main objectives of this study are to identify the spatial-temporal distributions of gaseous elemental mercury (GEM) in air and dissolved gaseous mercury (DGM) in surface seawater, and then to estimate the Hg(0) flux. The GEM concentration was lower in summer (1.61 ± 0.32 ng m(-3)) than in fall (2.20 ± 0.58 ng m(-3)). The back-trajectory analysis revealed that the air masses with high GEM levels during fall largely originated from the land, while the air masses with low GEM levels during summer primarily originated from ocean. The spatial distribution patterns of total Hg (THg), fluorescence, and turbidity were consistent with the pattern of DGM with high levels in the nearshore area and low levels in the open sea. Additionally, the levels of percentage of DGM to THg (%DGM) were higher in the open sea than in the nearshore area, which was consistent with the previous studies. The THg concentration in fall was higher (1.47 ± 0.51 ng l(-1)) than those of other open oceans. The DGM concentration (60.1 ± 17.6 pg l(-1)) and Hg(0) flux (4.6 ± 3.6 ng m(-2) h(-1)) in summer were higher than those in fall (DGM: 49.6 ± 12.5 pg l(-1) and Hg(0) flux: 3.6 ± 2.8 ng m(-2) h(-1)). The emission flux of Hg(0) from the ECS was estimated to be 27.6 tons yr(-1), accounting for ∼0.98% of the global Hg oceanic evasion though the ECS only accounts for ∼0.21% of global ocean area, indicating that the ECS plays an important role in the oceanic Hg cycle.

Mercury distribution, methylation and volatilization in microcosms with and without the sea anemone Bunodosoma caissarum

Mercury (Hg) has a complex biogeochemical cycle in aquatic environments. Its most toxic form, methylmercury (MeHg), is produced by microorganisms. This study investigated how the sea anemone Bunodosoma caissarum affects Hg distribution, methylation and volatilization in laboratory model systems. (203)Hg was added to microcosms and its distribution in seawater, specimens and air was periodically measured by gamma spectrometry. MeHg was measured by liquid scintillation. After the uptake period, specimens had a bioconcentration factor of 70 and in microcosms with and without B. caissarum, respectively 0.05% and 0.32% of the initial spike was found as MeHg. After depuration, MeHg in specimens ranged from 0.2% to 2.4% of total Hg. Microcosms with B. caissarum had higher Hg volatilization (58%) than controls (17%), possibly due to Hg(2+) reduction mediated by microorganisms associated with its tissues and mucus secretions. Marine organisms and their associated microbiota may play a role in Hg and MeHg cycling.

Elemental mercury (Hg(0)) in air and surface waters of the Yellow Sea during late spring and late fall 2012: concentration, spatial-temporal distribution and air/sea flux

The Yellow Sea in East Asia receives great Hg input from regional emissions. However, Hg cycling in this marine system is poorly investigated. In late spring and late fall 2012, we determined gaseous elemental Hg (GEM or Hg(0)) in air and dissolved gaseous Hg (DGM, mainly Hg(0)) in surface waters to explore the spatial-temporal variations of Hg(0) and further to estimate the air/sea Hg(0) flux in the Yellow Sea. The results showed that the GEM concentrations in the two cruises were similar (spring: 1.86±0.40 ng m(-3); fall: 1.84±0.50 ng m(-3)) and presented similar spatial variation pattern with elevated concentrations along the coast of China and lower concentrations in the open ocean. The DGM concentrations of the two cruises were also similar with 27.0±6.8 pg L(-1) in the spring cruise and 28.2±9.0 pg L(-1) in the fall cruise and showed substantial spatial variation. The air/sea Hg(0) fluxes in the spring cruise and fall cruise were estimated to be 1.06±0.86 ng m(-2) h(-1) and 2.53±2.12 ng m(-2) h(-1), respectively. The combination of this study and our previous summer cruise showed that the summer cruise presented enhanced values of GEM, DGM and air/sea Hg(0) flux. The possible reason for this trend was that high solar radiation in summer promoted Hg(0) formation in seawater, and the high wind speed during the summer cruise significantly increased Hg(0) emission from sea surface to atmosphere and subsequently enhanced the GEM levels.

Mercury in the Mediterranean, part I: spatial and temporal trends

The present paper provides an overview of mercury studies performed in the Mediterranean Sea region in the framework of several research projects funded by the European Commission and on-going national programmes carried out during the last 15 years. These studies investigated the temporal and spatial distribution of mercury species in air, in the water column and sediments, and the transport mechanisms connecting them. It was found that atmospheric concentrations of Hg compounds, particularly oxidised Hg species observed at five coastal sites in the Mediterranean Sea Basin, are significantly higher compared with those recorded at five coastal sites distributed across N Europe, most probably due to natural emissions. Hg levels in water are comparable to other oceans. Anthropogenic and natural point sources show locally limited enrichments, while natural diffusive sources influence Hg speciation over larger areas. Results and statistic comparison of mercury species concentrations within Mediterranean compartments will be presented and discussed.

The cycling and sea-air exchange of mercury in the waters of the Eastern Mediterranean during the 2010 MED-OCEANOR cruise campaign

An oceanographic cruise campaign on-board the Italian research vessel Urania was carried out from the 26th of August to the 13th of September 2010 in the Eastern Mediterranean. The campaign sought to investigate the mercury cycle at coastal and offshore locations in different weather conditions. The experimental activity focused on measuring mercury speciation in both seawater and in air, and using meteorological parameters to estimate elemental mercury exchange at the sea-atmosphere interface. Dissolved gaseous mercury (DGM), unfiltered total mercury (UTHg) and filtered total mercury (FTHg) surface concentrations ranged from 16 to 114, 300 to 18,760, and 230 to 10,990pgL(-1), respectively. The highest DGM, UTHg and FTHg values were observed close to Augusta (Sicily), a highly industrialized area of the Mediterranean region, while the lowest values were recorded at offshore stations. DGM vertical profiles partially followed the distribution of sunlight, as a result of the photoinduced transformations of elemental mercury in the surface layers of the water column. However, at some stations, we observed higher DGM concentrations in samples taken from the bottom of the water column, suggesting biological mercury production processes or the presence of tectonic activity. Moreover, two days of continuous measurement at one location demonstrated that surface DGM concentration is affected by solar radiation and atmospheric turbulence intensity. Atmospheric measurements of gaseous elemental mercury (GEM) showed an average concentration (1.6ngm(-3)) close to the background level for the northern hemisphere. For the first time this study used a numerical scheme based on a two-thin film model with a specific parameterization for mercury to estimate elemental mercury flux. The calculated average mercury flux during the entire cruise was 2.2±1.5ngm(-2)h(-1). The analysis of flux data highlights the importance of the wind speed on the mercury evasion from sea surfaces.

Dissolved gaseous mercury production in the dark: Evidence for the fundamental role of bacteria in different types of Mediterranean water bodies

It is well established that the dissolved gaseous mercury (DGM) production in waters is mainly driven by photochemical processes. The present paper provides evidence for a significant bacteria-mediated DGM production, occurring also under dark conditions in environmentally different types of coastal water bodies of the Mediterranean basin. The DGM production was laboratory determined in sea, lagoon-brackish and lake water samples, comparing the efficiency of the DGM production processes in darkness and in the light. This latter condition was established by exposing samples at solar radiation intensity in the Photosyntetical Active Radiation region (PAR) of 200 W m(-2). Mercury reduction rate in the dark was of the order of 2-4% of the DGM production in lightness, depending on the total mercury concentration in the water, rather than the bacterial abundance in it. Support for the active bacterial role in mercury reduction rate under dark conditions was provided by: 1) absence of significant DGM production in sterilized water samples (following filtration treatment or autoclaving), 2) restored DGM production efficiency, following re-inoculation into the same water samples of representatives of their bacterial community, previously isolated and separately cultured. Notwithstanding the low bacteria-mediated vs. the high photo-induced DGM production, whatever natural water body was considered, it is worth stressing the significant contribution of this organismal-mediated process to oceanic mercury evasion, since it occurs continuously along the entire water column throughout the 24 h of the day.

Importance of the biogenic organic matter in photo-formation of dissolved gaseous mercury in a culture of the marine diatom Chaetoceros sp

Laboratory experiments on DGM production under light/dark cycles in a culture of the marine diatom Chaetoceros sp. spiked with 200 ng l(-1) of mercury have been performed. DGM formation has been investigated also in the cell exudates, obtained by filtration of the cell culture. Results show that the cell culture and the filtrate give the same value of DGM production (2.24+/-0.88 pg min(-1) l(-1) and 2.23+/-0.02 pg min(-1) l(-1), respectively) in the light (40 W m(-2)), values much higher than to those obtained in the medium culture alone. A significant DGM production has been measured in dark conditions both in the cell culture (0.48+/-0.11 pg min(-1) l(-1)) and in the filtrate (0.85+/-0.10 pg min(-1) l(-1)). The results highlight that the organic compounds released by the cell in the culture medium play a fundamental role in the DGM photo-formation processes.